Chromium plating medium for a portable plating device

ABSTRACT

Desirable electrodeposition of bright chromium plate can be made onto a substrate from a portable plating device having an absorbent mass containing chromic plating medium. The medium contains a complex, water-soluble chromic compound for plating, which compound contains carboxylic acid constituents and halogen constituents. A uniform, bright chromium plate can be readily obtained on the substrate at a desirable current density that avoids the problems accompany ying portable plating at elevated temperature.

United States Patent [191 Bride [111 3,725,214 Apr. 3, 1973 [54] CHROMIUM PLATING MEDIUM FOR A PORTABLE PLATING DEVICE [75] Inventor: John Edwin Bride, Mentor, Ohio [73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

22 Filed: Feb. 19,1971

211 Appl.No.: 117,126

[52] US. Cl. ..204/l5, 204/51, 204/224 [51] Int. Cl ..C23b 5/48, C23b 5/06, B23p 1/02 [58] Field of Search ..204/15, 51, 224, 16

[56] References Cited UNITED STATES PATENTS 5/1956 Icxi...

2,748,069 Icxi t.

3,393,134 7/1968 Schwartz.....

3,006,823 10/1961 Deyrup 3,311,548 3/1967 Brown 3,021,267 2/1962 Berzins ..204/51 3,282,812 11/1966 Brown ..204/5l 3,475,294 10/1969 Seyb ..204/5l 3,475,295 10/1969 Smith ..204/5l 3,461,048 8/1969 Mahlstedt ..204/41 Primary Examiner-John H. Mack Assistant Examine -T. Tufariello AttorneyRoy Davis, William A. Skinner and John J. Freer [57] ABSTRACT 4 Claims, No Drawings CHROMIUM PLATING MEDIUM FOR A PORTABLE PLATING DEVICE BACKGROUND OF THE INVENTION Decorative chromium plating from portable plating devices using chromium plating solutions containing chromium in the trivalent state and in association with carboxylic acid and amine constituents has offered promise for commercial use. U.S. Pat. No. 2,748,069,

for example, describes such an electrolytic plating medium which is particularly useful for plating small work pieces, i.e., spot plating. However, in using such a medium for spot plating with a portable plating device, current densities of about 1,500 to about 3,000 amperes per square foot (ASF) are used and the tempera tures of the plating medium may be as high as the boiling point of same.

The necessitation of this practice has detracted from the broad acceptance of such a process, and further since especial portable plating devices, having heat dissipation means, must be used; also, plating solutions at a temperature approaching their boiling point do not readily lend themselves to use in a manually operable plating technique, particularly from the view point of the operator. Moreover the heat generated during plating at very high current densities can cause streaking of the plate, occurring when the deposit is burned as a result of too rapid depletion of plating medium from the absorbent mass. Hence, to maintain a balanced set of chemical and temperature conditions in the cathode film, requires careful operator control.

SUMMARY OF THE INVENTION It has now been found that bright chromium plate can be readily deposited on a substrate from a portable plating device while obviating problems associated with elevated temperature. The uniform, bright chromium plate can be deposited with excellent freedom from streaking and at. current densities that offer freedom from, or desirable alleviation of, problems of heat dissipation arising at higher current densities. Such operation can be achieved with a portable plating device having an absorbent mass containing a medium supplied with a complex, water-soluble chromic compound for the deposition of chromium plate, with this compound containing carboxylic acid constituents and halogen constituents. The plating achieved by such operation can lay down a bright, attractive decorative chromium coating, e.g., over a freshly plated bright nickel substrate. The medium supplied to such absorbent mass shows a bright range from 950 ASF down to 3 ASP on a 10 amp, 3 minute modified I-Iull cell panel.

Broadly the invention is directed to the electrodeposition of bright chromium plate onto a substrate from a portable plating device having an absorbent mass containing chromic plating medium, with the medium being supplied with a complexfwater-soluble chromic compound for the depositionof chromium plate. This compound contains carboxylic acid constituents and halogen constituents selected from the group consisting of chlorine, fluorine, bromine, iodine and mixtures thereof, with the complex having a molar concentration of chromium within the range from about 0.5 to about 3. e

The invention is further directed to the method of chromium plating an article with a decorative chromium plate and onto such an article from a portable plating device, and further relates to the electroplated article thereby obtained.

5 DESCRIPTION OF THE PREFERRED EMBODIMENTS The electroplating medium contains some to all of a water-soluble chromic compound containing carboxylic acid constituents plus halogen constituents which can be chlorine, fluorine bromine, iodine or mixtures thereof. However, in typical commercial plating opera" tion, bromine and iodine are often not used, for economy and to avoid evolution of visible noxious fumes at the anode. Therefore chlorine and fluorine are almost exclusively use.

Although the water-soluble chromic compound need not have acid constituents representative of only an especial group of carboxylic acids, such acids which can or have been used for the chromic compounds are typically exemplified by monocarboxylic and dicarbox ylic acids, with or without hydroxyl. groups. For plating efficiency and water-solubility, advantageously the acids are non-aromatic acids containing less than about l0 carbon atoms; representative acids include glycolic acid, formic acid, oxalic acid and their mixtures. For enhanced plating performance, the plating medium used virtually always contains a chromic compound which has carboxylic acid constituents supplied at least in major amount by glycolic acid. The compound of any of these acids such as a salt or ester thereof, which acts in any of the reactions such as those disclosed in more detail hereinbelow whereby the complex is formed, in the same manner as the free acid, can be used. Y

One method is the straightforward combination of chromium metal with carboxylic acid plus hydrochloric acid. When such combination includes particulate chromium metal to reduce reaction time, the reaction can be highly exothermic, and therefore caution needs be taken in carrying out same. Typically for enhanced reaction efficiency, as the reaction proceeds and the evolved heat starts to diminish, external heating is applied; and, where the reaction proceeds in aqueous medium such external heating can involve refluxing of the reaction mixture to augment completion of the reaction.

The complex of this type may also be prepared from the carboxylic acid and hydrochloric acid in admixture with chromic acid, typically charged to the reaction medium as a solution of chromic acid in water. The chromic acid can be supplied by any of the suitable substances for forming chromic acid in water, e.g., chromium trioxide. The reaction resulting from this method is also exothermic and caution in the use of such method is also thus advisable. Although these methods are not meant to be exhaustive concerning complex preparation, these complexes may further be prepared by reaction of chromic halide, with such halide corresponding to the halide that is to be present in the complex, which chromic halide is reacted with the carboxylic acid, this reaction further involving the addition of strong base, e.g., an alkali metal hydroxide. For example, CrF -9I-I,O may be used in this method and will readily yield a chromium/carboxylic acid/fluoride complex involving exothermic reaction conditions.

These carboxyl containing complexes virtually always contain a molar ratio of chromium atoms to carboxylate constituents within the range of 1:0.7. to l:3.0. Where halogen is present the complex essentially always has a molar ratio of chromium atoms to halogen atoms within the range of 1:01 to 113.5. Especially preferred ratios, based upon desirable plating performance and economy can depend upon the acid and also upon the halogen constituent when such is present. Thus for example, for a chromic carboxylate complex prepared with glycolic acid, the ratio of the chromic ion to the glycolic is preferably maintained within the range from about 121.1 to 1:2.1. For a complex containing a substantial amount of the glycolic acid for the carboxylate, which complex further contains chloride as the major amount, to all, of the halogen, the ratio of- The complex is generally present in the bath in an I amount to provide from about to about 150 grams of chromium per liter, that is, the molar concentration of chromium in the plating medium is within the range from about 0.5 to about 3. The complex is virtually always present in a bath wherein the liquid medium is supplied simply by water, plus a minor contribution from additional substances in liquid form. These additional substances can include excess acid used in the preparation of the complex.

However, typically for enhancing the chromium rate of deposition from the bath in the high current density area, minor amounts of organic additives can be added. These may be, added in liquid form, thereby contributing to the liquid medium of the bath. Such additives include polar aprotic substances which can be cyclic or acyclic organic materials or their mixtures. Representative of these additives are dimethyl formamide, tetrahydrofuran, dimethylsulfoxide, and mixtures thereof. Although some of all of these may form more than a very minor portion of the liquid medium, water, for economy, will supply the preponderant amount of such medium. These organic additives for enhancing the chromium rate of deposition in the high current density area may also be various ether, thioethers, glycol hydroxy ethers, such compounds that also contain carboxyl groups, and their mixtures. These ethers and other substances should have more than four carbon atoms per molecule and have an atomic ratio of oxygen to carbon atoms within the range of 0.25:1 to 0.9: l.

The bath may also contain a sulfite component, for enhancing the chromium rate of deposition in the low current density area typically after considerable working of the bath, which component can be contributed by at least one compound, where such exists, of a metal sulfite, or metal bisulfite or metal meta bisulfite, or trialklammonium bisulfite as well with mixtures of these. The substances may be added to the bath or formed in situ and those that are particularly preferred are the alkali metal and alkaline earth metal sulfites and bisulfites, e.g., sodium bisulfite.

The bath can further contain a salt of a strong acid preferably, for economy, an alkali metal salt; these salts enhance the conductivity achieved in the electroplating operation. Most preferably, for economy, the cation of the salt is sodium, potassium or their mixtures, and the strong acid anions should be halide anions, from an acid having a dissociation constant of at least K=l0' for example chloride. The plating medium usually contains between about 50-200 grams per liter of such salts, basis liters of water in the plating medium. Such medium can also contain boric acid, or an equivalent to boric acid in aqueous solution, such as borax, boron oxide, or sodium oxyfluoborate. Such compounds may operate in the plating medium to augment the rate of deposition of the chromium and are typically used in amounts between about 10-70 grams per liter of the medium, basis liters of water in the medium.

Before deposition of chromium, the bath pH is adjusted to within a range depending upon the complex present, but typically is adjusted to a pH within the range from about 1.8 to 4.9. However, for example, a complex containing a substantial amount of fluorine as the halogen is preferably maintained at a slightly more elevated bath pH than for a bath where chlorine supplies the major amount of the halogen. Such adjustment of pH can be readily carried out with a base, particularly alkali metal carbonates or hydroxides, with sodium or potassium hydroxide or their mixtures being preferred. Before addition to the bath, such material for adjusting the bath pH can be initially dissolved in water and the water solution then added to the bath.

The temperature of the medium during plating is in part dependent upon the amount of the medium on the substrate and upon the current density used for the plating. Since the spot plating of this invention proceeds at a relatively low current density the temperature of the medium on the substrate does not tend to reach an elevated temperature and thus the risk of forming a deleterious, burned deposit, is reduced or eliminated. Typically the temperature of the medium on the substrate during plating will be below about 50 C. and preferably for enhanced freedom from plating of a burned deposit, will be below about 40 C.

During plating, the object to be plated is made the cathode and the anode is supplied by the portable plating device. Such a device can be a manually operable plating device that may have a continuous feed of plating medium to the device. For example U.S.Pat. No. 2,961,395 shows a portable plating device that in one aspect has a roller with absorbent material mounted on the roller and means for supplying a stream of plating medium to the roller. The plating device may suitably also be one which is repeatedly dipped in plating medium in between applications of such medium to the substrate. For example a brush or stylus for such dipping can have a graphite block serving as the anode, which block is preferably padded with an acid-resistant felt type pad contained in stretchable acid-resistant, woven sock that holds the pad to the anode stylus surface. The anode stylus assembly can then be repeatedly dipped into the plating medium.

- The substrates for plating include metal such as steel, brass, copper, copper alloys including bronze, nickel and plated zinc, diecastings. Additionally such plating can be performed on plastic surfaces which are activated or prepared for electroplating operation.

The following example shows a way in which the invention has been practiced but should not be construed as limiting the invention. Unless otherwise specified, plating tests in the example is conducted in a modified Hull cell to determine the bright plating range of the medium to be used for the spot plating. The standard I-Iull cell is a trapezoidal box of non-conductive material and at the opposite ends of which are positioned anode and cathode plates, and as has been more particularly described in U.S. Pat. No. 2,149,344. For either the standard or the modified Hull cell, it is possible to easily determine the effective plating range of a plating composition under varying conditions. The current density at any point on the cathode is determined according to the formula A=C(27.748.7 log L) wherein A is the current density in ASP at the selected point, C is the total current in amps applied to the cell and L is the distance in inches of the selected point from the high current density end of the plate.

In the modified version of the Hull cell used herein, r-inch holes are introduced in the parallel sides of the cell adjacent the anode and cathode whereby, upon immersion of the cell in another vessel containing plating solution, into which vessel the cell will fit very closely, improved electrolyte circulation and consequent improved temperature control is afforded, as more particularly described in an article appearing in Plating, Volume 46, Number 3, (1959), Page 257.

EXAMPLE Into a container there is placed 0.8 mole of chromium metal, 1.8 moles of glycolic acid of 70 percent strength, that is, 70 weight percent of glycolic acid in a balance of water, and 0.5 mole of 37.3 percent strength hydrochloric acid which is 37.3 percent by weight I-ICI in water. The container is covered and good ventilation is provided. After the ingredients are placed together in the container, dissolution of the chromium starts slowly but gradually increase this supplying heat to the reaction. As the reaction continues the temperature of the reaction medium reaches 71 C. without external heating and the chromium can be seen by visual observations to be substantially dissolved. As the temperature starts to subside from 71 C., external heating is applied and the temperature of the reaction medium is permitted to reach 88 C. until all the chromium has dissolved. Thereupon the solution is heated at reflux, reaching a temperature of 107 C., for about 2 hours, and is thereafter permitted to cool.

The resulting solution is cooled and has a molar ratio of chromium atoms to glycolic acid of 1:225 and of chromium to chloride of. 110.625. Sufficient of this resulting solution is then charged to a mixing tank, to provide 40 g./l. of chromium. There is then added 150 g./l. of potassium chloride 63 g./l. boric acid, and about milliliters per liter of bis-(2-methoxyethyl) ether. The pH of the bath is then adjusted to about 2.9 to 3.1 by the addition of a sodium hydroxide solution in water. Additional water is used to bring the volume of the bath to 1000 milliliters. This bath is then electrolyzed ata rate of about to 30 amp hours then ready for modified I-lull cell testing to determine readiness of the solution for plating at optimum performance.

In the cell tests graphite anodes are used and the cathode is a 3 15/16 inches X 2 5/8 inches brass panel that is nickel coated prior to use in the cell. Hull cell tests are carried out at a bath temperature of 97 F. and at 14 volts using 10 amperes current for a 3 minute cycle. From such tests the bright range of decorative chrome plating is seen to be from about 950 ASF down to 3 ASF, and the solution is judged to be ready for use in'spot plating.

Brass panels, 4 inches by 2.5 inches in size, are prepared for plating by cathodic electro cleaning, rinsing, acid dip activation in either dilute l-ICl or H and subsequent rinsing. For the brush plating a commercially available stylus is used. This is composed of a graphite block which is padded with a felt pad made from a commercially available fibrous polyethylene terephthalate, which pad is then covered with an acid resisting cloth. The stylus is made the anode and the brass panels are the cathode.

Each panel is spot plated by repeatedly dipping the stylus portion of the plating device onto the abovedescribed plating medium and manually brushing the freshly dipped stylus across the surface of the panel in short circular strokes. The plating medium is maintained at about F. and the stylus operates at 1-.l .5 amps, and 4 volts. For each panel, the surface is first completely wetted by the padded stylus when no current is flowing; following this, the brush plating application is initiated.

The procedure is repeated to apply a bright chromium coat to the following substrates: buffed steel, nickel plated brass, and buffed copper. After plating, all panels are visually inspected and each is seen to have a smooth, uniform plate free from streaks and dark areas, each panel exhibiting a bright, decorative chromium plate. These results obtained under the abovedescribed conditions are regarded as providing a highly desirable spot plating system offering attractive com mercial potential. For best results the freshly plated nickel substrate, whether it be, for example, on steel, copper or bronze, should be receptive to chromium electrodeposition with the absence of any interferring isolated passivation areas which could possible cause gray patches under the chrome plate.

I claim:

1. In the method of electrodepositing bright chromium plate onto a substrate from a portable plating device having an absorbent mass containing chromium plating medium, the improvement which comprises supplying said mass with plating medium containing a sulphite component, an organic component selected from the group consisting of polar aprotic substances, ethers, thioethers, glycol hydroxy ethers, and their mixtures, and a compound for the deposition of chromium plate that is i a complex, water-soluble, trivalent chromic compound containing halogen constituents selected from the group consisting of chloride, fluoride, mixtures thereof and mixtures. thereof with other halides, which compound contains carboxylicacid constituents supplied by acids selected from the group consisting of glycolic, oxalic, and mixtures thereof said medium having a molar concentration of chromium within the range from about 0.5 to 3 and having a molar ratio of chromium atoms to carboxyl constituent within the range of 1:0.7 to 1:3, and a molar ratio of chromium atoms to halogen atoms within the range of 120.1 to 1:3.5.

2. The method of claim 1 wherein said portable plat- 4. The method of claim 1 wherein water supplies lng device manually Operable and Said absorbent more than about volume percent of said medium, mass is made the anode during electrodeposition.

3. The method of claim 1 wherein said medium is maintained within a pH of between about l.8-4.9 and 5 at a temperature not substantially above about 50 C.

and said halogen is selected from the group consisting of chlorine and fluorine. 

2. The method of claim 1 wherein said portable plating device is manually operable and said absorbent mass is made the anode during electrodeposition.
 3. The method of claim 1 wherein said medium is maintained within a pH of between about 1.8-4.9 and at a temperature not substantially above about 50* C.
 4. The method of claim 1 wherein water supplies more than about 20 volume percent of said medium, and said halogen is selected from the group consisting of chlorine and fluorine. 